The invention relates to photoconductive relays composed of a light-emitting device and a photoconductive switching element, and, in particular, to a photoconductive relay suitable for use in microwave and other switching applications in which the photoconductive relay is structured to have enhanced performance, a reduced size and a lower manufacturing cost.
A small, very-high-speed photoconductive relay can be made by combining a photoconductive switching element with a light-emitting device such as a laser or a light-emitting diode. The photoconductive switching element includes a layer structure composed of one or more Group III-V semiconductors, typically GaAs or InP, having different band-gap energies. Two electrodes are located on the surface of the layer structure and are separated by a narrow gap. The portion of the surface of the layer structure exposed in the gap between the electrodes defines a light-receiving region. The photoconductive relay is switched by illuminating the light-receiving region of the photoconductive switching element with light generated by the light-emitting device.
The ON resistance of the photoconductive relay depends inversely on the intensity of the light that illuminates the light-receiving region. The generally-diverging light beam emitted by the light-emitting device must be accurately concentrated on the light-receiving region. Conventionally, lenses, optical fibers or a combination of both have been used to couple the light from the light-emitting device to the light-receiving region of the photoconductive switching element. Correctly aligning such elements relative to the photoconductive switching element and the light-emitting device at low cost and in large volumes has proven challenging, however.
Japanese Published Patent Application No. 6-18554 discloses a probe device for measuring high-frequency signals using a photoconductive switch. Pulses of light produced by a light source, which may be a semiconductor laser, for example, are directed by an optical system consisting of an optical fiber and a collimator lens to illuminate a photoconductive switching element. The photoconductive switching element is electrically connected to conductors located on the surface of a substrate, and controls the flow of signal current through the conductors. The above-mentioned patent application states that the need for a complex light transmission system can be obviated if a semiconductor laser is used as the light source and if the semiconductor laser can be mounted in direct contact with the substrate in the vicinity of the photoconductive switching element.
The above-mentioned published patent application also discloses a structure in which a photoconductive switching element is sandwiched between a measurement chip and a transparent electrode covering the end face of the core of an optical fiber.
As mentioned above, mounting the light source in direct contact with the substrate of the photoconductive switching element offers the possibility of reducing the overall size of a photoconductive relay. Because the light-receiving region of a high-speed photoconductive switching element is small, and because it is preferable to minimize the resistance between the terminals of the photoconductive switching element, i.e., the ON resistance, it is desirable for as much as possible of the light emitted by the light source to be concentrated into the light-receiving region. To help meet these objectives, the distance between the light source and the light-receiving region of the photoconductive switching element should be reduced and the in-plane accuracy with which the light source is positioned relative to the light-receiving region should be increased. Moreover, as the photoconductive relay is made smaller, the accuracy with which the in-plane positioning must be performed increases. However, it is also desirable for the manufacturing process to be simple. It should be possible to position the light source relative to the light-receiving region of the photoconductive switching element with the required accuracy rapidly and at low cost. Also, the number of photoconductive relays rejected for inaccurate positioning should be small.
It is also preferable for the materials surrounding the photoconductive switching element to have a low dielectric constant to reduce the inter-terminal capacitance of the photoconductive switching element in its OFF state, i.e., the OFF capacitance.
It is also preferable to minimize the cross-talk between the activation signal of the photoconductive relay, i.e., the current fed to the light source to cause it to emit light, and the information signal switched by the photoconductive relay.
Furthermore, heat generated by the activation current flowing through the light source should not impair the physical and electrical properties of the photoconductive relay.
Finally, the photoconductive relay should be capable of incorporating new, improved new light sources or photoconductive elements without incurring significant tooling costs.
The invention provides a photoconductive relay that comprises a light-emitting device, a photoconductive switching element and columns of a conductive, fusible material. The light-emitting device and the photoconductive switching element respectively include including a light-emitting region and a light-receiving region. The columns extend between the light-emitting device and the photoconductive switching element to locate the light-emitting region of the light-emitting device opposite the light-receiving region of the photoconductive switching element and separated from one another by a distance of no more than 100 xcexcm.
The invention also provides a method of making a photoconductive relay. In the method, a light-emitting device having a major surface including a light-emitting region and a photoconductive switching element having a major surface including light-receiving region are provided. Conductive pads are formed on the major surface of the light-emitting device and connecting pads are formed on the major surface of the photoconductive switching element. Bumps of a conductive, fusible material are formed on the conductive pads or the connecting pads. The light-emitting device and the photoconductive switching element are positioned with the major surfaces opposite one another and the bumps contacting the conductive pads or the connecting pads. The bumps are heated to melt them. The molten bumps move the light-emitting device or the photoconductive switching element relative to the other to align the light-emitting region and the light-receiving region, and to define the separation between the major surfaces. Finally the molten bumps are cooled to solidify them into columns that interconnect the light-emitting device and the photoconductive switching element with the separation no more than 100 xcexcm
The invention provides a compact, high-speed photoconductive relay in which the light-emitting region of the light-emitting device is mounted by columns at a defined, small separation from, and in accurate in-plane alignment with, the light-receiving region of the photoconductive switching element. This allows the light-emitting device to illuminate the light-receiving region of the photoconductive switching element directly with a high intensity, without the need to provide and align such elements as lenses and optical fibers. Also, the accurate, reproducible alignment provides a very high coupling efficiency, which reduces the ON resistance of the photoconductive switching element. The closely-mounted light-emitting device minimally increases the OFF capacitance of the photoconductive switching element. Furthermore, it is possible to use the structure and method described above to construct integrated circuits that include multiple high-performance photoconductive relays or a combination of one or more high-performance photoconductive relays and integrated electronic circuits.